Nonlinear optical (NLO) materials are used in electro-optic devices to effect efficient processing and transmission of information in optical communications. The NLO materials used in electro-optic devices include inorganic materials, such as lithium niobate, and organic compounds, such as hyperpolarizable organic chromophores.
Organic nonlinear optical materials characteristically provide advantageous properties associated with ultra-fast response times, low dielectric constants, high damage thresholds, and tailorability. The nonlinear optical response of these organic materials is often significantly greater than that of inorganic materials. Moreover, organic materials are considerably more readily fabricated into integrated device structures when used in polymer form.
In the manufacture of these integrated devices, nonlinear optical polymers are cast as films onto substrates by, for example, spin-coating from a solution of the polymer in a solvent. Typical substrate materials that are useful for electro-optic waveguides are inorganic materials such as silicon, gallium arsenide, gallium aluminum arsenide, and indium tin oxide. The fabrication of electro-optic devices incorporating nonlinear optical polymers generally includes the deposition of a plurality of layers of films onto a substrate. A typical electro-optic waveguide device includes a lower electrode layer, a lower cladding layer, an active guiding layer, an upper cladding layer, and an upper electrode layer. Such a device is fabricated by successively depositing and then drying and curing the layers.
The purpose of the cladding layers in an electro-optic device is to confine light in the active guiding layer and to isolate the guiding layer from poling and device operating electrodes. To confine light in the active guiding layer, the refractive index of the materials making up the cladding layer must be lower than that of the nonlinear optical materials of the guiding layer. The difference in the refractive indices must be relatively small to optimize device performance. The small refractive index difference is particularly important for electro-optic modulators that support only a single optical mode.
The nonlinear optical activity of an electro-optic device is optimized by applying an electric field that is localized across the active guiding layer and minimized across the cladding layers. This requires that the electrical resistivity of the materials making up the cladding layer are less than that of the materials making up the active guiding layer.
The optimization of electro-optic devices that utilize organic polymeric materials requires consideration of a variety of factors: control of relative refractive indices of the active guiding layer and the cladding layers; the desired high nonlinear optical activity of the guiding layer; the thermal, photo, and chemical stability of the active guiding layer; optical loss; control of relative electrical resistivities of the active guiding layer and the cladding layers; and the ease of fabrication in spin-coating multiple layers of materials onto a substrate.
One particular problem associated with optimizing the nonlinear optical activity of an active guiding layer in a multi-layered device is that during electric field poling at a temperature higher than the glass transition temperature of the active guiding layer, the conductivity of the cladding material intermediate the poling electrodes and active guiding layer reduces the voltage drop across the core. Maximized NLO activity of the core requires a greater applied poling voltage.
A need exists for cladding materials having increased conductivities, and preferably conductivity greater than that of the active guiding layer such that a greater percentage of the applied poling voltage is dropped across the active guiding layer resulting in a realization of a maximized NLO activity in the active guiding layer while minimizing applied poling voltage. The present invention seeks to fulfill this need and provides further related advantages.